前所未有 (qián suǒ wèi yǒu) is the most common and neutral idiom for 'unprecedented', used in both speech and writing. 空前 (kōng qián) is more compact and often used as an adjective (e.g., 空前的成就). 史无前例 (shǐ wú qián lì) emphasizes 'in history' and is more dramatic; 前无古人 (qián wú gǔ rén) stresses that no one in the past has done it, often for creative achievements.